High-strength hot-dip galvanized steel sheet with excellent ductility and formability and its manufacturing method

Abstract

The hot-dip galvanized steel sheet of the present invention has, by weight, 0.06 to 0.16% of carbon (C), 0.8% or less (excluding 0%) of silicon (Si), 2.1 to 2.7% of manganese (Mn), 0.4% or less (excluding 0%) of molybdenum (Mo), 1% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.02% or less of sulfur (S), 1% or less (excluding 0%) of aluminum (sol.Al), 0.001 to 0.04% of titanium (Ti), 0.001 to 0.04% of niobium (Nb), 0.01% or less (excluding 0%) of nitrogen (N), and boron. The steel sheet contains 0.01% or less of (B), 0.05% or less of antimony (Sb), the balance being Fe and other unavoidable impurities, and of the steel components in the base structure at 1 / 4t of the thickness of the base steel sheet, the components of C, Si, Al, Mn, Cr, Mo and B satisfy Relational Formula 1, and the microstructure of the base steel sheet is composed of, in area %, a total of 70% or more of bainite and tempered martensite, 10% or less of ferrite, the balance being fresh martensite and retained austenite, and the fraction of the retained austenite is 5% or less in area %.

Description

[Technical Field]

[0001] The present invention relates to the production of high-strength hot-dip galvanized steel sheets having a tensile strength of 980 MPa or higher, which are mainly used for automotive structural members, and more specifically to a hot-dip galvanized steel sheet having excellent ductility and formability, in which the relational expression YS×EL between yield strength (YS) and elongation (EL) is 9000 or higher, but the yield ratio (YS / TS) is 0.65 or higher, and a method for producing the same. [Background technology]

[0002] Recently, the automotive industry has been facing increasingly strict regulations aimed at protecting the global environment. As a result, fuel economy regulations have become stricter, necessitating the use of lightweight and high-strength steel sheets. Furthermore, regulations on crashworthiness to protect passengers have also expanded. To improve the crashworthiness of vehicle bodies, high-strength steel sheets with excellent yield strength have been adopted for structural components such as members, seat rails, and pillars. However, increasing the strength of steel sheets can lead to reduced ductility and formability. To address this issue, the development of materials that simultaneously satisfy high strength and high formability is required. Generally, increasing the strength of steel sheets leads to a decrease in elongation, which reduces workability, and therefore the development of materials that can address this issue is required. Conventional steel strengthening methods, such as solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening, have been studied. However, among the above methods, solid solution strengthening and grain refinement have the drawback of making it extremely difficult to produce high-strength steel with a tensile strength of 490 MPa or higher.

[0003] On the other hand, precipitation-hardened high-strength steel is a technology that ensures strength by precipitating carbonitrides through the addition of carbonitride-forming elements such as Nb, Ti, and V. The fine precipitates inhibit grain growth, resulting in finer grains and thus higher strength. While this technology has the advantage of easily achieving high strength at low manufacturing costs, it also has the disadvantage of requiring high-temperature annealing to induce sufficient recrystallization and ensure ductility because the fine precipitates rapidly increase the recrystallization temperature. Another drawback of precipitation-hardened steel, which is strengthened by the precipitation of carbonitrides in a ferrite matrix, is that it is difficult to obtain high-strength steel of 600 MPa or higher.

[0004] Various transformation-strengthened high-strength steels have been developed, including dual-phase (DP) steel, which is composed of a soft ferrite matrix and a hard martensite matrix; transformation-induced plasticity (TRIP) steel, which achieves high ductility by utilizing the transformation-induced plasticity of retained austenite; and complex-phase (CP) steel, which is composed of a composite structure of ferrite and hard bainite or martensite. Recently, automotive steel sheets have been required to have higher strength to improve fuel efficiency and durability. Demand for high-strength steel sheets with tensile strengths of 780-980 MPa or higher is increasing for body structures and reinforcements to ensure crash safety and passenger protection. Among these, DP steel sheets have excellent ductility and are the most commonly used automotive steel sheets. However, they suffer from low yield ratios (YR) and poor formability and workability. Furthermore, the trend toward increasingly high-strength steel sheets has led to cracks and wrinkles during press forming of automotive parts, making it difficult to manufacture complex parts. TRIP steel has a superior yield ratio and good workability compared to DP steel, but has the drawback of poor weldability due to the large amounts of Si and Al added to ensure high elongation.

[0005] To overcome these drawbacks of existing DP steel, careful heat treatment can be used to manufacture steel that meets a certain yield ratio while maintaining the high ductility of existing DP steel, allowing for the expanded application of high-strength steel to more complex parts. This can be achieved by utilizing Q&P (Quenching and Partitioning) heat treatment, a cutting-edge heat treatment technology that can secure retained austenite.

[0006] An example of a conventional technique for simultaneously ensuring the ductility and workability of the high-tensile steel sheet is the invention disclosed in Patent Document 1. In this technique, a considerable amount of austenite exists that cannot be stabilized by the Q&P temperature, and fresh martensite (FM) is formed in the final cooling stage. However, fresh martensite has a high carbon content, which inhibits hole expandability, so the heat treatment temperature must be carefully selected.

[0007] Another example of prior art is the invention disclosed in Patent Document 2. This technology provides a method for manufacturing cold-rolled steel sheets that utilize tempered martensite produced by quenching heat treatment to simultaneously achieve high strength and high ductility, and also have excellent sheet shape after continuous annealing. However, this technology has the problem that the carbon content is high at 0.2% or more, which results in poor weldability, and the Si content is also high at 1.0% or more, which can cause dents in the furnace during annealing.

[0008] The invention disclosed in Patent Document 3, which is a conventional technique, provides a method for producing a high-strength cold-rolled steel sheet having excellent hole expandability by quenching and reheating, but since the amount of Si added is as high as 1.3% or more, there is a possibility that dents may occur inside the furnace. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-177278 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-090432 [Patent Document 3] Korean Patent Publication No. 2016-0173006 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to provide a hot-dip galvanized steel sheet having excellent ductility and formability for use in automotive structural members, in which the yield strength (YS) and elongation (EL) relationship, YS×EL, is 9000 or more, while the yield ratio (YS / TS) is 0.65 or more, and a method for manufacturing the same.

[0011] However, the object of the present invention is not limited to the above-mentioned content. The object of the present invention can be understood from the entire content of this specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention. [Means for solving the problem]

[0012] One aspect of the present invention is In weight percent, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 2.1 to 2.7%, molybdenum (Mo): 0.4% or less (excluding 0%), chromium (Cr): 1% or less (excluding 0%), phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less, aluminum (sol.Al): 1% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.01% or less, antimony (Sb): 0.05% or less, the balance being Fe and other unavoidable impurities. Among the steel components in the base structure at the 1 / 4t point of the thickness of the base steel plate, the components of C, Si, Al, Mn, Cr, Mo and B satisfy the following relational expression 1, The present invention relates to a hot-dip galvanized steel sheet with excellent ductility and formability, in which the microstructure of the base steel sheet is composed of, by area %, 70% or more of bainite and tempered martensite combined, 10% or less of ferrite, and the remainder being fresh martensite and retained austenite, with the fraction of the retained austenite being 5% or less by area.

[0013] [Equation 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

[0014] The hot-dip galvanized steel sheet has a hole expansion ratio (HER) of 30% or more, and a yield strength (YS) / elongation (EL) relationship (YS×EL) of 9000 or more, while having a yield ratio (YS / TS) of 0.65 or more.

[0015] Another aspect of the present invention is a step of providing a steel slab that satisfies the above compositional components and Relational Formula 1, and then reheating the steel slab; hot rolling the reheated slab so that the exit temperature of the finish rolling is Ar3 to Ar3 + 50 ° C, and then coiling it at 400 to 650 ° C, and then cooling it to room temperature at an average cooling rate of 0.1 ° C or less; A step of cold-rolling the cooled hot-rolled steel sheet at a rolling reduction of 40 to 70% to produce a cold-rolled steel sheet; A step of continuously annealing the cold-rolled steel sheet at a temperature of 820 to 860 ° C.; a step of primarily cooling the continuously annealed steel sheet to a temperature range of 630 to 680°C at an average cooling rate of 10°C or less, and secondarily cooling the steel sheet to a temperature of 300 to 350°C at an average cooling rate of 5°C or more using hydrogen gas, and then reheating the steel sheet to a temperature of 400 to 480°C and holding the temperature for 60 seconds or more; The method for producing a hot-dip galvanized steel sheet excellent in ductility and formability includes a step of hot-dip galvanizing the held steel sheet at a temperature of 400 to 450°C, and then cooling the steel sheet to a temperature of Ms to 100°C or lower at an average cooling rate of 5°C or higher.

[0016] The microstructure of the hot-dip galvanized steel sheet may be composed, by area percentage, of 70% or more of bainite and tempered martensite in total, 10% or less of ferrite, and the remainder being fresh martensite and retained austenite, with the fraction of retained austenite being 5% or less by area percentage.

[0017] The method may further include a step of subjecting the produced hot-dip galvanized steel sheet to alloying heat treatment. [Effects of the Invention]

[0018] As described above, the present invention has a useful effect in producing high-strength hot-dip galvanized steel sheets that have a superior yield-tensile ratio (YS / TS) compared to conventional DP steels while maintaining the high ductility characteristic of DP steels by optimizing the chemical composition and manufacturing process. This prevents processing defects such as cracks that occur during press forming, enabling them to be used in a variety of automotive structural components with complex shapes that require high formability. Furthermore, it has the advantage of simultaneously ensuring the material quality and plating properties. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a diagram showing the change in the relational expression YS×EL between yield strength (YS) and elongation (EL) depending on the yield ratio (YS / TS) in an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing changes in the relational expression YS×EL between yield strength (YS) and elongation (EL) according to Relational Expression 1 in an embodiment of the present invention. [Figure 3] FIG. 2 is a diagram showing changes in hole expandability value according to Relational Formula 1 in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described below.

[0021] The inventors have confirmed that optimizing the steel composition and manufacturing process to introduce retained austenite, ferrite, bainite, and fresh martensite into the final microstructure can increase the yield ratio and improve workability compared to conventional DP steels. Furthermore, the inventors have experimentally confirmed that such changes in the microstructure improve ductility by alleviating local stress and deformation concentration after necking and delaying the formation, growth, and coalescence of voids that cause ductile fracture. Furthermore, the inventors have experimentally confirmed that the formation of 5% or less retained austenite during final cooling further improves ductility. Based on these experimental results, the present invention was completed.

[0022] That is, the present invention reduces the fractions of ferrite and martensite compared to existing DP steels and introduces retained austenite and bainite, thereby increasing the yield ratio and ensuring workability compared to existing DP steels. Furthermore, during plastic deformation, a large number of mobile dislocations are formed around the retained austenite, which helps improve ductility. This precisely controlled dual-phase steel ensures ductility while maintaining a high yield ratio compared to existing DP steels. This makes it possible to produce high-tensile hot-dip galvanized steel sheets with excellent ductility and workability.

[0023] The hot-dip galvanized steel sheet of the present invention having excellent ductility and formability contains, by weight, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 2.1 to 2.7%, molybdenum (Mo): 0.4% or less (excluding 0%), chromium (Cr): 1% or less (excluding 0%), phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.02% or less, aluminum (sol.Al): 1% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, and nitrogen (N). : 0.01% or less (excluding 0%), boron (B): 0.01% or less, antimony (Sb): 0.05% or less, balance Fe and other unavoidable impurities, and of the steel components in the base structure at 1 / 4t thickness of the base steel plate, the components of C, Si, Al, Mn, Cr, Mo and B satisfy the following relational expression 1, and the microstructure of the base steel plate is composed of, in area %, the sum of bainite and tempered martensite: 70% or more, ferrite: 10% or less, balance fresh martensite and retained austenite, and the fraction of the retained austenite is 5% or less in area %.

[0024] Hereinafter, the alloy composition of the base steel sheet constituting the hot-dip galvanized steel sheet of the present invention and the reasons for limiting its content will be described first. Here, "%" indicates % by weight unless otherwise specified.

[0025] C: 0.06~0.16% Carbon (C) is a very important element added to strengthen the transformed structure. Carbon promotes the formation of hard martensite in dual-phase steels, improving strength. As the carbon content increases, the amount of martensite increases. However, if the carbon content exceeds 0.16%, the strength of martensite increases, but the strength difference with ferrite, which has a lower carbon concentration, becomes significant. This strength difference makes it easier for fracture to occur at the interphase interface during plastic deformation, resulting in reduced ductility and work hardening. Furthermore, poor weldability can lead to welding defects during customer part processing. On the other hand, if the carbon content is lower than 0.06%, it is difficult to achieve the desired strength.

[0026] Therefore, taking this into consideration, in the present invention, the carbon content is preferably limited to the range of 0.06 to 0.16%, and more preferably controlled to the range of 0.07 to 0.15%.

[0027] Si: 0.8% or less (excluding 0%) Silicon (Si) is a ferrite-stabilizing element that promotes ferrite transformation and contributes to the formation of retained austenite by promoting carbon concentration in untransformed austenite during the Q&P process. It also effectively increases the strength of ferrite through solid solution strengthening and reduces the hardness difference between phases, making it a useful element that ensures strength without reducing the ductility of steel sheets. However, if its content exceeds 0.8%, it can induce surface scale defects, adversely affecting the surface quality of the plating and further reducing weldability and phosphatability. Therefore, the upper limit for its addition is limited to 0.8%. It is more preferable to limit it to 0.7% or less.

[0028] Mn: 2.1-2.7% Manganese (Mn) refines grain size without reducing ductility, completely precipitates sulfur (S) in steel as MnS, prevents hot embrittlement due to the formation of FeS, and strengthens the steel. At the same time, in dual-phase steels, it lowers the critical cooling rate at which martensite is obtained, facilitating martensite formation. While manganese content less than 2.1% can make it difficult to achieve the strength targeted in this invention, manganese content exceeding 2.7% can lead to problems with weldability and hot rolling, unstable material properties due to excessive martensite formation, and the formation of Mn-bands (Mn oxide bands) within the structure, increasing the risk of processing cracks and sheet fracture. Furthermore, manganese oxides can dissolve on the surface during annealing, significantly impairing galvanic properties. Therefore, in this invention, the Mn content is preferably limited to 2.1-2.7%, more preferably 2.3-2.5%.

[0029] Mo: 0.4% or less (excluding 0%) Molybdenum (Mo) is an element that delays the transformation of austenite to pearlite and refines ferrite, improving strength. Mo has the advantages of improving the hardenability of steel, finely forming martensite at grain boundaries, and controlling the yield ratio. However, Mo is an expensive element, and a higher Mo content increases manufacturing costs, resulting in cost disadvantages. Therefore, it is preferable to appropriately control the Mo content. To achieve the above effects, a maximum of 0.4% is preferably added. If the Mo content exceeds 0.4%, the alloy cost increases sharply, reducing economic viability, and the excessive grain refinement and solid solution strengthening effects can actually reduce the ductility of the steel. Therefore, in the present invention, the Mo content is limited to 0.4% or less, excluding 0% in consideration of the amount unavoidably added during manufacturing. More preferably, the Mo content is controlled to 0.3% or less.

[0030] Cr: 1% or less (excluding 0%) Chromium (Cr) is an element added to improve the hardening ability of steel and ensure high strength. It also plays a very important role in the formation of martensite, minimizing the decrease in elongation compared to the increase in strength, and is advantageous for manufacturing dual-phase steel with high ductility. In particular, Cr during the hot rolling process 23 The Cr content of Cr forms Cr-based carbides such as C6, some of which dissolve during annealing and some of which remain undissolved. This allows the amount of solute C in martensite to be limited to an appropriate level after cooling. This suppresses yield point elongation and is advantageous for producing dual-phase steels with low yield ratios. However, if the Cr content exceeds 1%, not only does this effect saturate, but there are also problems such as poor cold rolling due to an excessive increase in hot rolling strength, and the increased fraction of Cr-based carbides causes coarsening, resulting in coarsening of martensite size after annealing, leading to reduced elongation. Therefore, in the present invention, it is preferable to limit the Cr content to 1% or less, excluding 0% in consideration of the amount unavoidably added during manufacturing. More preferably, the Cr content is controlled to 0.6% or less.

[0031] P: 0.1% or less (excluding 0%) Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect, improving in-plane anisotropy and ensuring strength without significantly impairing formability. However, excessive addition of P significantly increases the likelihood of brittle fracture, potentially causing slab breakage during hot rolling, and acts as an element that impairs the surface properties of the coating. Therefore, in the present invention, the maximum P content is limited to 0.1%. However, 0% is excluded to take into account the level of unavoidable addition.

[0032] S: 0.02% or less (excluding 0%) Sulfur (S) is an unavoidably added impurity element in steel, and since it reduces ductility and weldability, it is important to control its content as low as possible. In particular, since it increases the possibility of hot shortness, it is preferable to control its content to 0.02% or less. However, 0% is excluded in consideration of the level that is inevitably added during the manufacturing process.

[0033] Sol.Al: 1.0% or less (excluding 0%) Acid-soluble aluminum (sol. Al) is an element added to refine the grain size and deoxidize steel, and like Si, it is a ferrite-stabilizing element. It distributes carbon in ferrite to austenite, improving martensite hardening ability and forming retained austenite. Furthermore, when maintaining the bainite region during annealing, it effectively suppresses carbide precipitation in bainite, thereby improving the ductility of steel sheets. However, if its content exceeds 1.0%, although it is advantageous for increasing strength due to its grain refinement effect, it increases the possibility of surface defects in plated steel sheets due to excessive formation of inclusions during continuous casting operations, and also leads to increased production costs. Therefore, in the present invention, it is preferable to control the sol. Al content to 1.0% or less.

[0034] ·Ti, Nb: 0.001~0.04% each Titanium (Ti) and niobium (Nb) are elements that are effective in increasing the strength of steel sheets and refining grain size through the formation of nanoprecipitates. When these elements are added, they combine with carbon to form very fine nanoprecipitates. These nanoprecipitates strengthen the matrix and reduce the hardness difference between phases. If the Ti and Nb contents are less than 0.001%, it is difficult to ensure these effects. However, if the Ti and Nb contents exceed 0.04%, manufacturing costs increase and ductility may be significantly reduced due to excessive precipitates. Therefore, the Ti and Nb contents are preferably limited to 0.001 to 0.04%, and more preferably, to 0.005 to 0.02%.

[0035] N: 0.01% or less (excluding 0%) Nitrogen (N) is an element that effectively stabilizes austenite, but if its content exceeds 0.01%, there are problems such as a sharp increase in the cost of refining steel and a significant increase in the risk of cracks occurring during continuous casting due to the formation of AlN, so it is preferable to limit its upper limit to 0.01%. However, 0% is excluded in consideration of the level at which unavoidable addition occurs.

[0036] ·B: 0.003% or less Boron (B) is a component that delays the transformation of austenite to pearlite during the cooling process during annealing, and is a hardening element that suppresses the formation of ferrite and promotes the formation of martensite. However, if the B content exceeds 0.003%, excessive B will concentrate on the surface, which may lead to deterioration of plating adhesion, so the B content is controlled to 0.003% or less. More preferably, the B content is controlled to 0.002% or less.

[0037] ·Sb: 0.05% or less Antimony (Sb) is distributed at grain boundaries and slows the diffusion of oxidizing elements such as Mn, Si, and Al through the grain boundaries, thereby suppressing surface oxide segregation and inhibiting coarsening of surface segregated oxides due to temperature increases and changes in the hot rolling process. However, if the Sb content exceeds 0.05%, not only does this effect saturate, but manufacturing costs and workability deteriorate as well. Therefore, the Sb content is limited to 0.05% or less. More preferably, the Sb content is controlled to 0.03% or less.

[0038] In addition to the above components, the present invention preferably comprises the balance Fe and other unavoidable impurities.

[0039] Next, the hot-dip galvanized steel sheet of the present invention can improve workability by increasing the yield ratio compared to existing DP steels while maintaining ductility. To achieve this, in addition to the above-mentioned alloy composition, it is necessary to satisfy the following control conditions for the microstructure and phase fraction of the base steel sheet. The fraction and distribution of the microstructure and the concentration of elements in the microstructure will be explained below.

[0040] The microstructure of the hot-dip galvanized steel sheet of the present invention is composed, in area percentage, of 70% or more of bainite and tempered martensite in total, 10% or less of ferrite, and the remainder being fresh martensite and retained austenite, with the fraction of the retained austenite being 5% or less. If the total of bainite and tempered martensite is less than 70% or the ferrite exceeds 10%, there is a problem in that the desired yield ratio cannot be ensured. Furthermore, there is a problem in that the content of Si and Al must be increased in order to make the retained austenite exceed 5%.

[0041] In the hot-dip galvanized steel sheet of the present invention, the components of C, Si, Al, Mn, Cr, Mo and B among the steel components in the base structure at the 1 / 4t point of the thickness of the base steel sheet satisfy the following relational expression 1.

[0042] [Equation 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

[0043] The present invention makes it possible to manufacture a hot-dip galvanized steel sheet having a yield strength (YS)-elongation (EL) relationship (YS × EL) of 9000 or more and a yield ratio (YS / TS) of 0.65 or more. To achieve this, it is important to control the contents of C, Si, Al, Mn, Cr, Mo, and B in the steel composition of the base structure at the 1 / 4t point of the steel sheet thickness so that they satisfy the above-mentioned relationship (1). Si and Al are ferrite stabilizing elements that promote ferrite transformation and contribute to the formation of retained austenite and martensite by promoting C enrichment in untransformed austenite. C also contributes to the formation and adjustment of martensite fraction by promoting C enrichment in untransformed austenite. On the other hand, Mn, Cr, Mo, and B are elements that contribute to improving hardenability, but their effect on C enrichment in austenite is relatively weaker than that of C, Si, and Al. Therefore, it is very important to properly adjust the ratios of C, Si, Al and the other hardening elements Mn, Cr, Mo, and B.

[0044] If the value defined by the above relational expression 1 is 0.35 or less, as described above, the relational expression YS×EL of the yield strength (YS) and elongation (EL) according to the yield ratio (YS / TS) can be ensured to be 9000 or more, and further, the fraction of bainite and tempered martensite can be ensured to be 70% or more, and the hardness difference between the phases can be reduced to ensure a hole expandability value of 30% or more. On the other hand, if the value defined by the above relational expression 1 exceeds 0.35, the above-mentioned effects are lost.

[0045] This composite structure, in which ferrite, bainite, martensite, and retained austenite are simultaneously formed, allows each phase to be finely and uniformly dispersed, while reducing the hardness difference between the phases, resulting in a superior yield ratio and improved workability and formability compared to conventional DP steels. Furthermore, this change in the microstructure reduces the concentration of local stress and deformation after necking, thereby delaying the formation, growth, and coalescence of voids that cause ductile fracture, thereby improving ductility.

[0046] This makes it possible to provide a hot-dip galvanized steel sheet having a hole expansion ratio (HER) of 30% or more, a yield strength (YS) and elongation (EL) relationship (YS×EL) of 9000 or more, and a yield ratio (YS / TS) of 0.65 or more.

[0047] Next, a method for producing a hot-dip galvanized steel sheet excellent in ductility and formability according to the present invention will be described.

[0048] In order to improve the yield-tensile ratio (YS / TS) compared to conventional DP steels while maintaining the lean chemical composition and high ductility characteristics of DP steels, the present invention requires control of the structure and chemical composition and careful heat treatment. First, it is important to introduce a small amount of retained austenite. Retained austenite induces transformation-induced plasticity and helps improve the ductility of steel sheets. To introduce this retained austenite, the steel is rapidly cooled to a temperature below Ms to form partial martensite, and then immediately reheated to a temperature above Ms to undergo a partitioning process. This process results in the formation of a large amount of bainite, which stably distributes carbon and contributes to the formation of retained austenite in the final structure. Bainite also reduces the hardness difference between the ferrite and martensite phases. To ensure further ductility, the process of partially forming ferrite is also important. The ferrite fraction is controlled to below 10% by annealing in the single-phase region or just below the single-phase region, and a small amount of additional ferrite can be formed in the slow cooling section. This further improves ductility. Furthermore, by precipitating fine nano-precipitates in ferrite, the hardness difference between the phases can be further reduced, improving workability. Finally, by introducing a small amount of fresh martensite during final cooling, the desired strength can be ensured.

[0049] To achieve this, the method for producing a hot-dip galvanized steel sheet of the present invention includes the steps of: preparing a steel slab that satisfies the above-mentioned compositional components and Relational Formula 1, and then reheating the steel slab; hot-rolling the reheated slab so that the temperature at the outlet of finish rolling is Ar3 to Ar3+50°C, and then coiling the slab at 400 to 650°C, followed by cooling to room temperature at an average cooling rate of 0.1°C or less; cold-rolling the cooled hot-rolled steel sheet at a reduction of 40 to 70% to produce a cold-rolled steel sheet; and rolling the cold-rolled steel sheet at a reduction of 820°C. the process includes a step of continuously annealing the steel sheet at a temperature of Ms to 860°C, a step of primarily cooling the continuously annealed steel sheet to a temperature range of 630 to 680°C at an average cooling rate of 10°C or less, a step of secondary cooling using hydrogen gas to a temperature of 300 to 350°C at an average cooling rate of 5°C or more, a step of reheating to a temperature of 400 to 480°C and holding the temperature for 60 seconds or more, and a step of hot-dip galvanizing the held steel sheet at a temperature of 400 to 450°C, and then cooling to a temperature of Ms to 100°C or less at an average cooling rate of 5°C or more.

[0050] First, a steel slab having the above-described composition is prepared and then reheated. The slab reheating process is a process of heating the steel slab to smoothly carry out the subsequent rolling process and to sufficiently obtain the target physical properties of the steel sheet. The present invention is not particularly limited to such reheating conditions, and ordinary reheating conditions may be used. One example is reheating in a temperature range of 1100 to 1300°C.

[0051] Next, in the present invention, the reheated steel slab is subjected to finish hot rolling so that the exit temperature of the finish rolling is Ar3 to Ar3 + 50° C. In this case, the present invention is not limited to specific hot rolling conditions, and a normal hot rolling temperature can be used.

[0052] In the present invention, the finish hot-rolled steel sheet is then coiled in a temperature range of 400 to 650°C, and then cooled to room temperature at an average cooling rate of 0.1°C or less to produce a hot-rolled steel sheet in which carbides that serve as austenite nucleation sites are finely dispersed. By uniformly dispersing fine carbides through the hot rolling process, austenite is formed finely dispersedly as the carbides dissolve during annealing, and as a result, fine martensite can be uniformly dispersed after annealing.

[0053] In the present invention, the cooled hot-rolled steel sheet is then cold-rolled at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet.

[0054] The coiled hot-rolled steel sheet is pickled and then cold-rolled at a reduction of 40 to 70%. If the cold reduction is less than 40%, it is difficult to obtain the target thickness and also difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, there is a high possibility of cracks occurring at the edge of the steel sheet, which increases the load during cold rolling. Therefore, in the present invention, it is preferable to limit the cold reduction to 40 to 70%.

[0055] Next, in the present invention, the cold-rolled steel sheet is subjected to continuous annealing at a temperature range of 820 to 860°C. This continuous annealing process is intended to form ferrite and austenite simultaneously with recrystallization and distribute carbon. If the continuous annealing temperature is below 820°C, it is difficult to ensure a sufficient austenite fraction, and the desired fractions of martensite, bainite, and retained austenite cannot be ensured after annealing. On the other hand, if the temperature exceeds 860°C, productivity decreases and excessive austenite is formed, resulting in a significant increase in the fractions of bainite and martensite after cooling, which increases yield strength and reduces ductility, making it difficult to ensure high ductility. In addition, elements that reduce wettability in hot-dip galvanizing, such as Si, Mn, and B, become heavily concentrated on the surface, which can degrade the surface quality of the galvanized steel.

[0056] In the present invention, the continuously annealed steel sheet is primarily cooled to a temperature range of 630 to 680°C at an average cooling rate of 10°C or less, and then secondary cooled to a temperature of 300 to 350°C at an average cooling rate of 5°C or more using hydrogen gas, and then reheated to a temperature of 400 to 480°C and held for 60 seconds or more.

[0057] The continuously annealed steel sheet is primarily cooled to a temperature range of 630-680°C at an average cooling rate of 10°C or less, and then secondary cooled to a temperature range of 300-360°C at an average cooling rate of 5°C / s or more using a hydrogen quenching facility using hydrogen gas to introduce some fresh martensite.Then, the steel sheet is immediately reheated to a temperature of 400-480°C and held for 60 seconds or more to form bainite and concentrate carbon in the surrounding untransformed austenite.

[0058] It is very important to control the quenching temperature during the secondary cooling to 300-360°C, which is below the martensite formation temperature Ms. If the quenching temperature exceeds 360°C, the fraction of martensite formed initially will be very small, or martensite formation will be difficult, preventing smooth carbon partitioning and making it difficult to form the desired fraction of retained austenite during final cooling. On the other hand, if the quenching temperature is below 300°C, deterioration of the sheet shape and load on the equipment may occur.

[0059] It is also important to control the reheating temperature to 400-480°C, which is above the Ms temperature. If the reheating temperature is below 400°C, bainite does not form quickly enough, preventing smooth carbon partitioning. If the reheating temperature exceeds 480°C, less bainite is formed, resulting in an increased fraction of fresh martensite during final cooling. In other words, it is extremely important to carefully control the secondary quenching temperature and reheating temperature during Q&P annealing to form the desired microstructure.

[0060] In the present invention, the steel sheet thus held is subsequently subjected to hot-dip galvanizing at a temperature of 400 to 450°C, and then cooled at an average cooling rate of 5°C or more to a temperature of Ms to 100°C or less, thereby producing a final product in which fresh martensite is formed adjacent to bainite. At this time, temper rolling can be performed at a reduction rate of less than 1%, if necessary.

[0061] The present invention may further include a step of subjecting the hot-dip galvanized steel sheet produced as described above to an alloying heat treatment. [Example]

[0062] The present invention will be described in detail below with reference to examples.

[0063] (Example) A steel slab having the composition shown in Table 1 below was prepared. The steel slab was reheated to a temperature range of 1050 to 1250°C and then finish hot-rolled at a temperature of 950°C within the range of Ar3 to Ar3 + 50°C. The hot-rolled steel sheet was coiled at 400 to 650°C and cooled at a cooling rate of 0.1°C per second or less to produce a hot-rolled steel sheet. The hot-rolled steel sheet was pickled and then cold-rolled at a reduction of 40 to 70%. The cold-rolled steel sheet was then continuously annealed at the temperatures shown in Table 2 below and then subjected to a Q&P heat treatment under the conditions shown in Table 2 below. The QP heat-treated cold-rolled steel sheet was then hot-dip galvanized, then alloyed, and finally cooled to introduce fresh martensite and retained austenite. The hot-dip galvanized steel sheet was then subjected to a temper rolling of less than 1%.

[0064] The microstructure and mechanical properties of each steel sheet manufactured as described above were evaluated, and the results are shown in Table 3 below. Tensile tests were performed on each test piece in the L direction according to ASTM standards, and the tensile properties (tensile strength (TS), yield strength (YS), and elongation (El)) were evaluated. The microstructure fraction was determined by analyzing the matrix structure at the 1 / 4t point of the plate thickness of the annealed steel sheet. Specifically, after Nital corrosion, the fractions of ferrite, bainite, fresh martensite, and austenite were measured using an FE-SEM and an image analyzer. Hole expandability was also measured using a hole expandability tester.

[0065] [Table 1] *In the steel sheet composition in Table 1 above, N is contained as an impurity element within the range of 30 to 50 ppm.

[0066] [Table 2]

[0067] [Table 3] *In Table 3, F means ferrite, B means bainite, TM means tempered martensite, FM means fresh martensite, and RA means retained austenite.

[0068] As shown in Table 1-3 above, in the case of Invention Examples 1-6, in which the steel composition and manufacturing process conditions satisfy the requirements of the present invention, the relational expression YS×EL between yield strength (YS) and elongation is 9000 or more, and the yield ratio (YS / TS) is 0.65 or more, and it is clear that the material properties and workability of the steel sheet targeted by the present invention can be ensured.

[0069] In contrast, in Comparative Examples 1 to 10, in which the steel composition and manufacturing process conditions were outside the ranges of the present invention, or the fraction and occupancy ratio of the internal structure of the steel were outside the ranges of the present invention, the yield strength (YS)-elongation relationship (YS×EL) was less than 9000, or the yield ratio (YS / TS) was less than 0.65. Therefore, it was not possible to simultaneously ensure the strength, ductility, workability, and weldability of the steel sheet targeted by the present invention.

[0070] Specifically, in Comparative Example 1, although the steel composition was within the range of the present invention, the secondary cooling temperature was too low, resulting in a residual austenite fraction exceeding 5%, and the excessive cooling rate caused a load on the equipment.

[0071] In Comparative Example 2, although the steel composition is within the range of the present invention, the secondary cooling temperature is too high, so tempered martensite is not sufficiently formed and carbon partitioning does not occur, so the target yield ratio cannot be obtained.

[0072] In Comparative Example 3, although the steel composition was within the range of the present invention, the reheating temperature was too low, resulting in the formation of excessive tempered martensite and making it impossible to obtain the desired strength. In Comparative Example 4, the reheating temperature was too high, resulting in a low bainite fraction and a high fresh martensite fraction, making it impossible to obtain the target yield ratio.

[0073] Comparative Examples 5-10 were cases in which the steel composition and manufacturing process conditions were outside the ranges of the present invention. Specifically, in Comparative Examples 5-6, the steel composition was outside the ranges of the present invention, and the continuous annealing temperature and reheating temperature were outside the ranges of the present invention, resulting in excessive ferrite formation and the desired yield ratio being unobtainable. In Comparative Examples 7-8, the steel composition was outside the ranges of the present invention, and the secondary cooling temperature and reheating temperature were too high to obtain the target yield ratio. In Comparative Examples 9-10, the reheating temperature was too high to obtain the target yield ratio.

[0074] On the other hand, Fig. 1 is a diagram showing the change in the relational expression YS × EL between yield strength (YS) and elongation (EL) according to the yield ratio (YS / TS) in Examples of the present invention (invention steels 1-6 and comparative steels 5-10), Fig. 2 is a diagram showing the change in the relational expression YS × EL between yield strength (YS) and elongation (EL) according to relational expression 1 in Examples of the present invention (invention steels 1-6 and comparative steels 5-10), and Fig. 3 is a diagram showing the change in hole expandability value according to relational expression 1 in Examples of the present invention (invention steels 1-6 and comparative steels 5-10). In Figs. 1-3, invention steels 1-6 indicate invention steels corresponding to invention examples 1-6.

[0075] As described above, the detailed description of the present invention has been given with reference to the preferred embodiment, but it goes without saying that a person skilled in the art to which the present invention pertains can make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be determined by the claims below as well as equivalents thereof.

Claims

1. The alloy contains, by weight, 0.06 to 0.16% carbon (C), 0.8% or less (excluding 0%) silicon (Si), 2.1 to 2.7% manganese (Mn), 0.4% or less (excluding 0%) molybdenum (Mo), 1% or less (excluding 0%) chromium (Cr), 0.1% or less (excluding 0%) phosphorus (P), 0.02% or less sulfur (S), 1% or less (excluding 0%) aluminum (sol. Al), 0.001 to 0.04% titanium (Ti), 0.001 to 0.04% niobium (Nb), 0.01% or less (excluding 0%) nitrogen (N), 0.01% or less boron (B), and 0.02 to 0.05% antimony (Sb), with the balance being Fe and other inevitable impurities; Among the steel components in the base structure at a 1 / 4t point in the thickness direction of the base steel plate, the components of C, Si, Al, Mn, Cr, Mo, and B satisfy the following relational expression 1: A hot-dip galvanized steel sheet having excellent ductility and formability, wherein the microstructure of the base steel sheet is composed, in area percentages, of a total of 70% or more of bainite and tempered martensite, 3 to 10% of ferrite, and the remainder being fresh martensite and retained austenite, wherein the fraction of the retained austenite is 5% or less and the fraction of the fresh martensite is 9% or more. [Relationship 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

2. 2. The hot-dip galvanized steel sheet excellent in ductility and formability according to claim 1, wherein the hot-dip galvanized steel sheet has a hole expansion ratio (HER) of 30% or more, a relational expression YS×EL between yield strength (YS) and elongation (EL) of 9000 or more, and a yield ratio (YS / TS) of 0.65 or more.

3. a step of preparing a steel slab containing, by weight, 0.06 to 0.16% carbon (C), 0.8% or less (excluding 0%) silicon (Si), 2.1 to 2.7% manganese (Mn), 0.4% or less (excluding 0%) molybdenum (Mo), 1% or less (excluding 0%) chromium (Cr), 0.1% or less (excluding 0%) phosphorus (P), 0.02% or less sulfur (S), 1% or less (excluding 0%) aluminum (sol. Al), 0.001 to 0.04% titanium (Ti), 0.001 to 0.04% niobium (Nb), 0.01% or less (excluding 0%) nitrogen (N), 0.01% or less boron (B), and 0.02 to 0.05% antimony (Sb), the balance being Fe and other unavoidable impurities, and satisfying the following relational expression 1, and then reheating the steel slab; hot rolling the reheated slab so that the exit temperature of finish rolling is Ar3 to Ar3 + 50 ° C., and then coiling it at 400 to 650 ° C., followed by cooling it to room temperature at an average cooling rate of 0.1 ° C. / s or less; cold rolling the cooled hot-rolled steel sheet at a reduction rate of 40 to 70% to produce a cold-rolled steel sheet; A step of continuously annealing the cold-rolled steel sheet at a temperature of 820 to 860 ° C.; a step of primarily cooling the continuously annealed steel sheet to a temperature range of 630 to 680°C at an average cooling rate of 10°C / s or less, and secondarily cooling the steel sheet to a temperature of 300 to 350°C at an average cooling rate of 5°C / s or more using hydrogen gas, and then reheating the steel sheet to a temperature of 400 to 480°C and holding the temperature for 60 seconds or more; and a step of hot-dip galvanizing the held steel sheet at a temperature of 400 to 450°C, and then cooling the steel sheet to a temperature of Ms to 100°C at an average cooling rate of 5°C / s or more, wherein the microstructure of the hot-dip galvanized steel sheet is composed, in area %, of a total of 70% or more of bainite and tempered martensite, 3 to 10% of ferrite, the balance being fresh martensite and retained austenite, with a fraction of retained austenite being 5% or less and a fraction of the fresh martensite being 9% or more by area. [Relationship 1] (4×C+Si+Al) / (Mn+Cr+5×Mo+200×B)≦0.35

4. The method for producing a hot-dip galvanized steel sheet excellent in ductility and formability according to claim 3, further comprising a step of subjecting the produced hot-dip galvanized steel sheet to an alloying heat treatment.

5. The method for producing a hot-dip galvanized steel sheet excellent in ductility and formability according to claim 3, wherein the produced hot-dip galvanized steel sheet is subjected to temper rolling at a reduction rate of less than 1%.

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